Hydraulic action

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Hydraulic action is a form of erosion caused by the force of moving water currents rushing into a crack in the rockface. The water compresses the air in the crack, pushing it right to the back. As the wave retreats, the highly pressurised air is suddenly released with explosive force, capable of chipping away the rockface over time. Thus, the crack is gradually widened so the amount of compressed air increases like a balloon, and hence the explosive force of its release increases. Thus, the problem intensifies (a 'positive feedback' system). It helps the river to get lower (rejuvination). It is helped in part on cliffs by 'Freeze Thaw'.

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Cracks,joints and crevices within the channel can be widened and therefore, weakened when the air contained in them becomes compressed with water.Also, this explosive power can weaken the rockface so that it comes to a point where the crack falls away.